This informal CPD article ‘Clinical Reasoning in Musculoskeletal Biomechanics - Part 2: Primary and Secondary Shortenings and the Two Equations’, was provided by Dr. Mauro Lastrico, Physiotherapist at AIFiMM Formazione, an organisation recognised by the Italian Ministry of Health as an authorised CME provider. They offer organised training courses in the Mézières Method, a rehabilitative and postural approach.
Part 1 established the four constants of the musculoskeletal symptom and the distinction between local and referred symptom [6]. What remains to be clarified is the causal origin of shortening. This second part distinguishes primary from secondary shortenings, analyses viscero-somatic correlations, and reduces the multiplicity of clinical manifestations to two fundamental equations [1,3].
1. Viscero-Somatic Correlations
Visceral pathologies and dysfunctions may also produce somatic symptoms through the neurological organ-vertebra connections [10,30]. The symptoms manifest at the vertebral site or in the periphery, following the projections of the corresponding nerve roots [11,13]. Viscero-vertebral correlations are an important element of differential diagnosis, because a symptom of visceral origin may present with the same characteristics as a primary musculoskeletal symptom [8,13].
A clinical example illustrates the mechanism. A patient presents with knee pain; local physical examination reveals no articular or ligamentous alteration sufficient to justify it. The corresponding dermatomal vertebral connection shows a significantly altered configuration at L3-L4, with compression of the segments [10,30]. Treatment directed at decompression and at rebalancing the muscles acting on the segment produces only temporary reductions: the instability of the results suggests a visceral problem, since a colonic dysfunction, through its neurological connection, may keep L3-L4 under tension and be its origin [30]. Without the resolution of the primary visceral cause, work on the musculoskeletal system cannot be stable over time [8].
The differential diagnostic criterion rests on the response to treatment: if the problem is primarily vertebral, vectorial rebalancing is resolutive and stable; if it is of visceral origin, the improvements are temporary and the symptomatology tends to recur [9].
2. Primary and Secondary Muscle Shortenings
The distinction between primary and secondary shortenings orients the therapeutic strategy [1].
2.1 Primary Shortenings
Primary shortenings are determined by the three systems that use the muscle as final effector: the neurophysiological, the biomechanical and the psychosomatic [3]. Each acts through the increase in basal tone which, sustained over time, involves the connective component and produces the permanent residual shortening [12,16,27]. Clinically they represent the majority of the causes of the musculoskeletal symptom, and in these cases vectorial rebalancing may achieve their resolution [1,3]. The intervention acts on the analytical plane, rebalancing the individual joints by lengthening the muscles responsible for the specific deformations, and on the systemic plane, avoiding aggravations in other districts [6,17].
2.2 Secondary Shortenings
Secondary shortenings are the adaptive consequence of structural or functional alterations of other systems. The muscular system is not the origin of the problem but the route through which a local problem, by producing a regional imbalance, generates a systemic articular misalignment [8,15]. Symptoms may manifest locally or in distant skeletal regions [20]. In the stomatognathic system, an occlusal alteration produces a regional muscular activation in shortening which, through the hyoid connections, propagates to the cranio-cervico-scapular system [5,29]. A skeletal alteration with leg length discrepancy of a lower limb produces a pelvic obliquity: the muscles reorganise asymmetrically to maintain equilibrium, and the reorganisation ascends along the spine to the upper districts, producing, for example, a scapulo-humeral conflict [15,18].
The stomatognathic system
Secondary triggers may arise from any system: stomatognathic, with imbalances of the cranio-cervico-scapular system through swallowing [29]; skeletal, with leg length discrepancies and congenital or acquired alterations requiring permanent compensations [14]; visceral, through the organ-vertebra connections [10,30]; visual and auditory, with compensatory positioning of the head when cervical rotations or inclinations are held in order to see or hear better [11,28]; neurological, with hypertonia from central lesions or activation deficits from peripheral lesions [21,22]; and still other systems may act in an analogous manner. In such cases a multidisciplinary approach is necessary [8].
Identification relies on tests specific to the various systems — for the stomatognathic system, the diagnostic bite reveals the interference of occlusion on the muscular system [5,29] — on the organ-vertebra correlations [30], and on instrumental investigations [8]. The most relevant clinical signal, however, remains the response to treatment: in secondary shortenings vectorial rebalancing produces temporary improvements, the corrections are lost and the symptomatology recurs, a sign that a primary cause is still active [9,23].
3. The Multidisciplinary Approach
In secondary shortenings the intervention of the competent specialist is required to resolve the primary cause, whose resolution may be sufficient for the remission of the symptomatology [8]. In other cases, even once the primary cause has been removed, the chronic muscular shortenings persist; at this point vectorial rebalancing, no longer obstructed by the primary perturbation, may produce results that are stable over time [1,3]. In both conditions the physiological articular sequence is altered, both analytically and systemically: in primary shortenings the restoration of vectorial equilibrium may be resolutive, whereas in secondary shortenings the resolution of the specific cause is necessary, possibly followed by muscular rebalancing [4,6].
4. The Two Equations
The multiplicity of clinical manifestations may be reduced to two processes.
First equation — primary shortenings: absence of alterations in other systems, primary muscular shortenings, alteration of the articular sequence, mechanical conflicts, pathologies. The muscular system is the origin of the process, and the shortenings arise from the action of the three systems — neurophysiological, biomechanical and psychosomatic; resolution proceeds through vectorial rebalancing [2,3].
Second equation — secondary shortenings: alteration of other systems, secondary muscular shortenings, alteration of the articular sequence, mechanical conflicts, pathologies. The muscular system manifests a problem originating elsewhere — visceral, stomatognathic, skeletal, visual, auditory, neurological, etc. — and the shortenings are adaptive attempts in response to a primary alteration; resolution requires intervention on the primary cause [8,15].
Behind the various diagnostic labels — epicondylitis, low back pain, neck pain, etc. — lie local manifestations of one of these two processes [8]. The distinction is not merely theoretical: it determines the therapeutic approach and the prognosis. Understanding which equation is active avoids the prolonged treatment of secondary shortenings that cannot stabilise without the resolution of the primary cause and, conversely, the search for non-existent causes when the problem resides in the muscular system itself [9,25].
Conclusions
The four constants, the tools of integrated assessment, the viscero-somatic correlations and the two equations provide the clinician with a system for transforming observation into biomechanical diagnosis [6,8]. Understanding the underlying physical principles allows the interpretation of phenomena, the prediction of their evolution and the orientation of interventions that are stable over time [24,26]. Clinical reasoning structured in this way makes explicit and verifiable the path that leads from the sign to the cause [7]. Once the active equation has been identified, the operative strategy follows the coherent therapeutic sequence — reduction of Resistant Force in the dominant shortened muscles and subsequent consolidating strengthening — whose applicative modalities are the subject of the following contribution [1,19].
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References
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